Limulus polyphemus hepatopancreas and muscle were examined for enzymes of hexose phosphorylation in order to demonstrate and characterize a fructomannokinase activity similar to that seen in the arthropod Homarus americanus. Unlike lobster, the predominant hexokinase of Limulus hepatopancreas is the fructomannokinase, which readily phosphorylates fructose and mannose but has little ability to phosphorylate glucose. The quantitatively large catalytic capacity for phosphorylation of fructose and mannose prompted an investigation of possible direct utilization of the reaction products by enzymes other than hexosephosphate isomerases and glucose-6-phosphate dehydrogenase. Preliminary surveys of homagenates of Limulus hepatopancreas have revealed the existance of an NAD+ specific hexose-phosphate dehydrogenase which does not react with glucose-6-phosphate but does react with mannose-6-phosphate and fructose-6-phosphate. Partial purification of this enzyme has been achieved by ion exchange and gel filtration HPLC chromatography. The significance of this project lies in the identification in a multicellular organism of a possible alternate pathway for hexose phosphate metabolism.